1. Field of the Invention
The present invention relates to a compression method, an extension method and an image processing apparatus.
2. Related Art
Printers, and so on have a composition which stores image data written out by a rasterizing process once in a memory, and reads out from the memory one by one when required. Though a controller generates the image data by resolution of e.g. 600 dpi, it is possible to perform the resolution conversion from 600 dpi to 1200 dpi when printing and printout at a high resolution.
Also, when it is stored in the memory, generally, the compression process is performed to the image data and the amount of data is reduced. Conventionally, a BTC (Block Truncation Coding) method is provided as a compression method (For example, see Japanese Patent Laid-Open No. 11-164150). This is a method of performing coding in block unit composed of a plurality of pixels.
On the other hand, in a controller side, it is possible to generate image data of high resolution such as 1200 dpi. However, when the resolution is high, since it is necessary to increase the capacity of the memory, the cost becomes high. Though it is considered that the image data are stored after performing the resolution conversion from 1200 dpi to 600 dpi so as not to increase the capacity of the memory, the image quality deterioration will occur when the resolution conversion is merely performed.
Especially, with respect to an edge portion of a character or a line drawing, it is well known to lose sharpness because an original resolution of an image can not be maintained after performing the compression process or the resolution conversion.